1. Field of the Invention
The present invention concerns an improvement for a dynamic pressure bearing device used, for example, in information appliances or audio and video appliances and, more in particular, it relates to an improvement for the rotational accuracy and the durability.
2. Description of the Prior Art
There has been known a conventional dynamic pressure bearing device of this kind, for example, as shown in FIG. 4 of the appended drawings. This prior art discloses a dynamic pressure bearing device of a cylinder for a magnetic head used in a digital audio tape recorder (DAT), a video tape recorder (VTR) or the like, and it rotatably supports an upper cylinder 2 having a pair of heads 1 at the circumferential edge thereof.
That is, a shaft 4 stands vertically at the axial center of a lower cylinder 3. On the other hand, a housing 5 of the dynamic pressure bearing device is secured to the axial center of the upper cylinder 2. The housing 5 as a rotational body has a sleeve 6 and a thrust receiver 7 made of a steel ball attached to one end thereof, and it is rotatably fit to the shaft 4. The sleeve 6 has cylindrical radial bearing surfaces 8 formed on its inner diametrical surface at two axially spaced apart positions, while the thrust receiver 7 has a thrust bearing surface 9. On the other hand, a shaft 4 fit into the sleeve 6 has a radial receiving surface 10 opposed to the radial bearing surface 8 with a gap of a radial bearing and a thrust receiving surface 11 opposed to the thrust bearing surface 9 being in contact therewith. Lubricant pits 12 are disposed in the vicinity of the thrust bearing surface 9 and an air escape aperture 13 is disposed at a portion between the inner diametrical surface of the sleeve 6 and the thrust receiver 7 for communicating the lubricant pit 12 with the external atmosphere. Herringbone-shaped grooves 15 are formed to the radial receiving surface 10. The herringbone-shaped grooves 15 comprise circumferential rows of inner spiral groove segments 15A each disposed on the side between the radial bearing surfaces 8, 8 at two upper and lower positions and circumferential rows of outer spiral groove segments 15B each disposed on the side opposite thereto, that is, on the axially outer side.
The upper cylinder 2 is driven rotationally by an incorporated motor 20 around the shaft 4 as the center. When the upper cylinder 2 rotates, the pressure of a lubricant filled in the gap of the radial bearing is increased under the pumping effect of the herringbone-shaped groove 15 on the shaft 4, by which the sleeve 6 is supported radially to the shaft 4 being kept free from contact therewith. Simultaneously, the steel ball of the thrust receiver 7 is axially supported being in contact with the thrust receiving surface 11 at the upper end of the shaft 4.
The rotationally driving motor 20 comprises a cylindrical rotor magnet 21 attached to the lower surface of the upper cylinder 2 and a stator coil 22 attached in a face-to-face fashion to the inner circumferential surface of the lower cylinder 3. Further, a stationary cylindrical rotary transformer 23 disposed along the outer circumference of the sleeve 6 is attached to the lower cylinder 3. On the other hand, a rotatable rotary transformer 24 opposed in a face-to-face fashion to the outer circumferential surface of the stationary rotary transformer 23 is attached to the upper cylinder 2. Then, a signal taken out of a magnetic heads 1 attached to the upper cylinder 2 is transmitted by way of the rotatable rotary transformer 24 to the stationary rotary transformer 23.
When the stater coil 22 of the rotationally driving motor 20 is energized, a rotating force is generated in the rotor magnetic 21, and the sleeve 6 and each of accessory parts secured thereto are rotated as a unit.
The dynamic pressure bearing device used for a cylinder of a magnetic head as described above is required to have high rotational accuracy and long lasting durability.
However, in the conventional dynamic pressure bearing device involves a problem that a lubricant such as oil or grease filled in the gap of the radial bearing between the radial bearing surface 8 and the radial receiving surface 10 and in the lubricant pits 12 is gradually forced out and discharged from the gap of the radial bearing and the lubricant pits 12 during rotation, thereby bringing about the change of torque in the bearing to reduce the rotational accuracy and, in a worst case, causing scorching of the bearing.
Further, there is also a problem that the lubricant leaked and discharged through the gap of the bearing is scattered by centrifugal force to contaminate the magnetic heads 1 thereby bringing about a worry that the device can no more function correctly.
In view of the above, we have made various experiments trying to find out the causes for the discharge of the lubricant and, as a result, could confirmed the followings. That is, the groove pattern of the herringbone-shaped grooves 15 is usually formed as a symmetrical configuration. Accordingly, the axial length a for each of the outer groove segments 15B in the upper herringbone-shaped grooves 15 should be equal with the axial length b for each of the inner groove segments 15A in the upper herringbone-shaped grooves 15 (a=b), while the axial length c for each of the inner groove segments 15A should be equal with the axial length d for each of the outer groove segments 15B in the lower herringbone-shaped grooves 15 (c=d).
However, it may sometimes occur actually that the length of the inner groove segment 15A is greater than the length of the outer groove segment 15B (a&lt;b, c&gt;d) by a slight asymmetry due to fabrication error upon fabrication of the grooves. This brings about a so-called pumping out effect in which the lubricant filled in the bearing is gradually forced out of the sleeve 6 accompanying with the rotation of the sleeve 6.